And I'm Robert Siegel. In this segment, Syria, sarin and Israel. The Israelis have joined France and Britain in concluding that Bashar al-Assad's forces have used sarin, a lethal nerve agent, on Syrian rebels.

CORNISH: The U.S. has not formally reached that conclusion but in the past, President Obama has spoken of the possible use of chemical weapons as a red line, a potential game-changer, one that would presumably require a rethink of Washington's reticence over intervention in the Syrian civil war. In this segment, we'll go to the Golan Heights, which Israel took from Syria several decades ago.

SIEGEL: But first, these two questions: How can anyone be certain that sarin has been used? And if Washington were certain, what might that crossing of the red line lead to in the way of U.S. action? We'll take up that question first with Susan Glasser, who's the editor-in-chief of Foreign Policy magazine. Welcome to the program, once again.

SUSAN GLASSER: Thank you so much for having me.

SIEGEL: And if, indeed, it is established that sarin were used in Syria - some other nerve agent - and a red line has been crossed, what are the kinds of options that are on the table at the White House, to respond to that crossing of the red line?

GLASSER: First of all, I think Washington - and the White House - is very eager for that red line not to be established as having been crossed. So first of all, what they're doing, I think, is buying - for a certain amount of time and saying, we absolutely have not yet determined that these chemical weapons have been used, despite the statements of the Israelis, the British and the French. And until it's determined, we can't determine our response.

So that's number one - how much time can they put back on the clock by saying it's not determined. And then number two, even were that to be the case, is it going to be enough to overcome what is clearly a great personal reluctance of President Obama himself, to have any escalating involvement in Syria.

I think it's been very clear, over the course of more than two years, that the president has no real interest in launching a major military intervention in Syria; and doesn't seem to have a playbook ready in hand, for what to do in the case of something like a sarin-like gas being employed.

SIEGEL: There are proposals ranging from send in troops to seize the depots where these weapons are stored, if they're all known, to prevent them from either being used again or falling into other hand; airstrikes against those sites, or against particular units. How do you understand the reluctance? Is it the president saying, look, I just got us out of Iraq; I don't want another war? Or is it, we have Iran on our plate and that's our main concern; we can't get distracted? Or there are just no good options in Syria?

GLASSER: Yes. I think it's fair to call Syria the land of no good options, from the point of view of the White House these days, in part because of a real concern and worry that there are no good partners for the United States in that civil war; that the strongest fighting forces against the Assad regime seem to be extremist Islamist elements, possibly even allied with al-Qaida.

The other, more democratic-leaning groups don't seem to have the heft on the ground, and the military weight. But clearly, the last thing the United States wants to do is to get involved in another ground war in the heart of the Middle East, if President Obama can avoid it.

SIEGEL: Which means that this red line - to mix metaphors - is an extremely high bar. The U.S. is now saying, to say that that red line has been crossed is going to require a lot of proof.

GLASSER: Well, that's exactly right. And you could imagine a scenario where he says, listen, we need to have a whole new diplomatic process; we need a United Nations investigation. There are many ways, potentially, to push back the clock on the moment at which that red line is officially declared crossed.

SIEGEL: Susan Glasser, editor-in-chief of Foreign Policy magazine, thank you very much for talking with us.

GLASSER: Thank you.

SIEGEL: Sarin is a chemical weapon. Inhale sarin vapor; and according to a U.S. government report, without an antidote, you could die within a minute. Sarin gas was used by a Japanese cult in a Tokyo subway attack that killed 13 people. The Iraqis used sarin in a notorious attack on a Kurdish city, Halabja, inside Iraq during Iraq's war with Iran. Thousands of people died there.

So how can you tell if sarin, in fact, has been used? We're going to ask Charles Blair that. He's senior fellow for state and non-state threats, at the Federation of American Scientists. Welcome.

CHARLES BLAIR: Thank you so much for having me.

SIEGEL: The Israelis cite evidence of Syrians dying, frothing at the mouth with dilated pupils. A top military analyst there says that points to death by nerve gas, probably a liquefied nerve agent. Does it impress you?

BLAIR: No. I think it's significant that the accusation was made but at the same time - and also, it's significant because they identified a type of agent, a nerve agent. The French and British, similarly, have cited reports which they haven't released, that are - I would call vague.

SIEGEL: Those are supposed to be based on soil samples, that people have found something in the soil that would indicate that sarin had been used. I mean, are these reliable tests that such a toxic substance has been deployed?

BLAIR: Sarin is a non-persistent agent, meaning it evaporates very quickly. And, in fact, it evaporates so fast that the metro that was attacked in Tokyo opened up for business the next day. But at the same time, it does stick around long enough so that if you transport victims - say, in an ambulance - without knowing that they have sarin on their clothing, there is a chance of secondary contamination.

With sarin, you can check for it in the soil, and you can also check for it on material. And so in soil, it lasts between two and 24 hours. It's temperature-dependent between about 40 and 75 degrees. So that's not a very long time frame for somebody to collect the sample, get it out, and have it analyzed.

SIEGEL: The Israeli report seems to be based either on pictures, or eyewitness testimony about the way people have died.

BLAIR: Yes. That's...

SIEGEL: Are there other things that could kill people in a way that it would look like that, in a field hospital?

BLAIR: Yeah, there's several other agents that are - possibly could have that effect. There are what are called riot-control agents and also, incapacitants; and those are typically, not lethal. The U.S. uses them; domestic law enforcement uses them.

SIEGEL: Various forms of tear gas or pepper gas, you're saying.

BLAIR: Exactly. So if these - some of the symptoms are consistent with that; and they could have been delivered in a manner, and in a concentration, that actually caused death.

SIEGEL: Not the same thing as using a weapon of mass destruction, like sarin.

BLAIR: Precisely. Does the United States want to go to war over the use of tear gas? The answer is, obviously, no.